1. Field of the Invention
This invention relates to stainless steel having good workability and atmospheric corrosion resistance that is provided with a highly-adhesive chrome coating applied to the surface thereof by means of physical vapor deposition (PVD) and which also has good decorative compatibility with wet-type chromium plating.
2. Description of the Prior Art
Stainless steel has long been used extensively as a corrosion-resistant material that can be used as is, without any surface treatment. However, use in a severe environment, such as a coastal area, leads to staining and the formation of red rust on the surface of the stainless steel which results in a loss of the superb surface appearance that has always been a reason for selecting stainless steel.
It is desirable that stainless steel materials, especially those for exterior use, be capable of retaining their superb surface appearance indefinitely and that the surface quality be protected for a long period of time. From this viewpoint, it is necessary to improve the atmospheric corrosion resistance of the stainless steel.
Increasing the chromium component or adding elements such as nickel or molybdenum in order to increase the corrosion-resistance of the stainless steel itself has been employed and various highly corrosion-resistant stainless steels have been developed. However, the above methods have had the drawbacks that they increase the manufacturing costs and it is difficult to ensure the requisite tensile strength and toughness.
It is well known that chromium is an element that plays a fundamental role in the high corrosion-resistance of stainless steel. The corrosion-resistance is further improved by a method that involves chrome-plating the surface of the stainless steel by means of a wet-type process. However, inasmuch as the stainless steel surface is thus covered with a passive film providing the surface with the said corrosion-resistance, there have been problems involving the difficulty of applying the chrome plating and the adherence and uniformity thereof. Because of this, pretreatment has been employed such as cathodic electrolysis or pickling in an acid solution immediately prior to the plating. Also, in order to prevent passivation of the stainless steel surface and increase the adhesion by absorbing, in whatever degree, deformation of the plated chrome, a lamination method has been put forward in Japanese Patent Publication No. 50(1975)-35010 consisting of the deposition of a thin film of copper on the surface in an acid solution containing copper ions, followed by a layer of nickel and a layer of chrome.
However, because in each of the aforementioned wet-type plating methods the stainless steel is pickled in the plating solution, i.e. an aqueous solution, perfect prevention of passivation of the stainless steel is difficult, so that no matter how much effort is expended additional processing is still required, causing a problem that peeling readily occurs. Stainless steel used for building materials, kitchen equipment and the like may, when the work is being executed, be extensively shaped at edge portions or joints of roofs, window frames and the like. When the processes in which stainless steel may be used in practice are considered, an essential condition is that it possess excellent adhesion that will not give rise to peeling of the chrome film even when the stainless steel is shaped. Also, with wet-type chrome plating, degradation of the plating solution occurs during the plating process. As a consequence, impurities in the plating solution can become entrained in the chrome film, leading to deterioration in the performance of the chrome film.
With stainless steel now being used more extensively for kitchen equipment and piping around toilets and the like, decorative compatibility with the existing wet-type chrome plating has become required. This means that it is desired that the stainless steel surface color tone and luster be identical with those of chrome plating.